I enjoyed the Enders Game series as well as Cards side series that followed the exploits of Bean and the rest of the students left behind when Ender went off to his destiny. I haven't got ahold of the last book of the series yet "The Shadow of the Giant" I think it is called. I went to buy it at a book store but it will not be soon that I will pay $12 for a paperback. The rest I found ebook editions of on the newsgroups so eventually I will find this last one.
If you are up for a challenging read have a look a "Dhalgren" by Samuel Delany. It comes in around 600 pages and has caused wars to happen at SF conventions--or at least heated debates.
On a lighter note I enjoyed Harry Harrisons "Deathworld" trilogy -- pure space opera.
I also recommend John Brunner. "The Sheep Look Up," "The Shockwave Rider," and of course "Stand On Zanzibar." Ursula K. Leguins "The Dispossessed" was a good read with a fairly mature theme (scholarly, not "Adult".) Also her Earthsea series was good although it may not seem like much nowadays.
Piers Anthony was good before he decided to become a formula author (similar to Stephen King who stinks

) Get ahold of Macroscope, the Manta series ("Omnivore", "Orn" and "Ox") the Battle Circle trilogy ("Sos the Rope", "Var the Stick" and "Neq the Sword". The Xanth series is light reading if you like puns. After that he got so prolific that it became impossible to keep up.
Andre Nortons Witchworld series was good. It is targeted at a younger audience.
Larry Niven is good for hard SF, particularly the "Ringworld" series.
I could go on...